Retinal image in axial and refractive ametropia.

THERE are some features relative to the size of the retinal images in these two conditions which are of general interest, though they are rarely discussed. Axial Ametropia.-The " basic " (blurred) retinal image is smallest in hyperopia, intermediate in emmetropia, and largest in myopia. The size of a blurred retinal image is best taken from the points in the diffusion circles at the extremities of the image where the nodal ray strikes the retina. We call this the " basic " retinal image. The increase or decrease in size of the " basic " image over its size in emmetropia is about 2 per cent. for every dioptre of error. When the correcting lens is placed at F1 of the eye, the resultant clear retinal image in hyperopia and myopia is the same size as that in emmetropia. Refractive Ametropia.-The " basic " retinal image is similarly smaller in hyperopia and larger in myopia than that in emmetropia. The decrease or increase in size is much less than in the axial type, being about 0 5 per cent. for every dioptre of error. In a medium grade of refractive error, say 4 D, this will amount to about 2 per cent. This statement is contrary to the general teaching ofmost texts, but it can be proved by calculating the position of the nodal point in refractive ametropia.